docs: def unique_event_labels(event_list):
      """Find the unique event labels
  
      Parameters
      ----------
      event_list : list or dcase_util.containers.MetaDataContainer
          A list containing event dicts
  
      Returns
      -------
      list
          Unique labels in alphabetical order
  
      """
  
      if isinstance(event_list, dcase_util.containers.MetaDataContainer):
          return event_list.unique_event_labels
  
      else:
          labels = []
          for event in event_list:
              if 'event_label' in event and event['event_label'] not in labels:
                  labels.append(event['event_label'])
  
          labels.sort()
          return labels